Vision in vertebrate retinas is initiated by a light-activated G protein coupled pathway located on disc membranes in the photoreceptor cells. Signaling through this pathway involves the formation and activation of a variety of membrane-dependent protein complexes. We are studying the structures of these complexes as they exist on membrane surfaces through electron crystallography. We have formed two-dimensional crystals and helical arrays of the heterotrimeric form of the G protein and imaged them by cryo-electron microscopy. Image processing of the helical structures has revealed layer lines in the computed Fourier transform to beyond 18 , and indexing of these transforms is in progress.